Method of manufacturing aluminum



Sept. 10, 1963 H. A. SOUTHGATE 3,103,453

METHOD OF MANUFACTURING ALUMINUM SHEATH FOR ELECTRIC CABLE Filed Sept. 6. 1960 FIG. I

FIG.2

FIG.3

ATTOKQJEYS United States Patent 3,103,453 1 METHGD OF MANUFACTURING ALUMINUM SHEATH FOR ELECTRIC QABLE Harold Arthur Southgate, Millihen, Untario, Canada, as-

signor to Canada Wire and Cable Company Limited, Toronto, Ontario, (3311263, a corporation Filed Sept. 6, 1969, Ser- No. 54,018

5 Claims- (Cl. 148-115) This invention relates to a method for the manufacture of aluminum sheath for electric cables and, in particular, to a method which enables certain disadvantages of the prior art to be overcome.

Aluminum sheathed cable is commonly made by drawing an insulated conductor through an aluminum tube, the inside diameter of which is such that the insulated conductor may be pulled through. The aluminum tube is then drawndown to tightly embrace and grip the insulated conductor by running the sheathed cable through a die to reduce the diameter of the tubing.

In the manufacture of aluminum sheath for use in covering electric cables it is common practice to extrude aluminum tubing from aluminum billets. each aluminum billet contains only suiiicient metal to form a predetermined length of tube and, when the material in the billethas been extruded it is necessary to reload the press with a further billet so that the extrusion process may continue.

When extruding tubing of relatively small diameter for use as a sheath on a small electrical cable it is possible, owing to the physical nature of the press to insert a second billet in the machine in a relatively short period of time. In one particular example a press manufacturing, by an extrusion process, small aluminum tubing can be reloaded with a fresh billet in approximately seventeen seconds.

In large presses, however, making extruded tubes of, for example, two inches inside diameter it takes between two and three minutes to reload the press with a fresh billet.

This delay in the extrusion process of a given length of tubing gives rise to very serious difliculty when the tubing is used as a sheath on an electrical cable. I It has been iouind that the delay which occurs during-the time that a press is being reloaded with a fresh billet causes a metallurgical discontinuity in the extruded tube which, although not visible to the naked eye, seriously afiects the physical properties of the tube. It has been found that the portion of the tube which has been extruded and which lies immediately adjacent to the outlet of the extrusion die grows very large crystals during the time that the press is being reloaded whereas the portion of the tube immediately within the extrusion'die has a minute, granular, crystalline structure. ferential line of division between the granular section and the section containing the large crystals.

Electrical cables having metallic sheaths must, in accordance with certain regulations, be capable of passing tests with regrad to their bending properties (among many of the properties which are tested). The bending test requires that the sheathed cable be bent through 180 1 around a reel which has a diameter varying with the diameter of the cable. The cable must then be straightened and bent in the reverse direction through 180 around the same diameter reel. This bend and reverse bend must be completed twice. g

If this aluminum sheath, as it comes from the manufacturing plant, is used, it has been found that the sheath fractures along the circumferential line of division in There is a very definite circumhas been forced to manufacture aluminum sheathed cable in lengths which incorporate an aluminum sheath which has been extruded from-a single billet since the discontinuity in the metallic structure of the tube has, heretofore, made it impossible for the sheathed cable to meet the necessary requirements to make it acceptable foruse. As an alternative,'under special circumstances, this discontinuity has been mechanically reinforced. This has proved unsatisfactory in that the mechanical reinforcement does not always prevent damage to the'sheath even when special care is taken in handling the finished cable.

In the cable making art and in the aluminum tubing extrusion tart, the metallurgical discontinuities as referred to above are known as press stops or die stops. In the balance of this specification we will refer to these metallurgical discontinuities by the terms recognized in the trade. i

'In order to overcome the disadvantage of the press stops it was first attempted to out out the press stops and tobutt-weld the tubing together so as to form a continuous length of "tube of a length greater than Obviously,

precise extent towhich the diameter of the tubing willbe reduced during the swaging-operation will vary in accord-' the crystal structure of the metal or Very close thereto during this bending test. Accordingly, the cable industry could be extruded from a single billet. This practice is not satisfactory for a number of reasons and, in effect, the cable makingindustry has been limited in that it is unable to produce an acceptable continuous length of cable which exceeds the length'of tubing which can be extruded from a single billet. v

' 'It is the object :of the present invention to-providea method of treating a continuous aluminum tubing which contains one or more press stops in such a manner that these press stops are no longer objectionable and have imparted tothem metallurgical characteristics which enable them to withstand the bending tests which the cable must pass in order to be commercially acceptable and which the cable will encounter in actual use. I

Certain metallurgical properties of the-cable are illustrated in the accompanying photomicrographs in which FIGURE 1 illustrates a section cut from an extruded aluminum tube and including a press stop in the untreated state; 1 p

FIGURE 2 is a view of the same material: shown in FIGURE 1 after it has been subjected to a 5% elongation \anda subsequent annealing step, and,

FIGURE 3*is a view of the sameymaterial after it sheath and drawing the sheath down to tightly embrace the core. Preferably, the tube is cold-worked by drawing it down to a smaller diameter. :It has been found that a material improvement is observed itthe tube is reduced in diameter by drawing and subsequently annealed.

Preferably, however, the diameter of the tubing'is reduced within the rangexof between 4 /2 and 2.0%; The

once with certain considerations. Firstly, the swaging of the tube work-hardens the aluminum and if the tubing is swaged down too much, the material lying between adja-' cent press stops will, in the finished product, be work-hardened to an undesirable extent. Further, too great 2. reduc tion in diameter will necessitate/the commencement of the operation with a tube of a diameter much larger than would otherwise be necessary in order to accommodate the cable which it is intended to'contain. This will, accordly, increase the cost due to the higher cost of the large di- I ameter tubing with which the process must commence.

If the tubing is not reduced in diameter to a suificient extent, however, the full benefits of the invention will not be achieved. Accordingly, a compromise is reached which reduces the undesirable eflects of work-hardening the aluminum and which, at the same time, enables the advantages of the invention to be achieved to the optimum degree. The specification will include specific examples which it is believed be sufficient to enable one skilled in the art to clearly comprehend the factors which should be taken into account.

The annealing step is carried out subsequent to the straining or drawing down of the tube and, conveniently, is carried out over only a limited length of the tube, the length being that, of course, which contains the press stop which it is desired to treat. Preferably, the annealing step will involve raising the temperature of the area containing the press stop within the range of between 600 and l,000 F. and holding it at that temperature for a period varying from 30 minutes to five hours depending on the temperature used. Aluminum melts at a temperature of approximately 1,220 F. and, of course, care must be taken to ensure that the temperature does not rise to a point where the metal deteriorates.

Referring to the photomicrographs it will be seen, that, in FIGURE 1, the line of division indicated generally by reference character 10 is clearly visible between the area which has been extruded and the amorphous area which, during the period'of time and the press was being reloaded, lay within the die. FIGURE 2 discloses the same piece of material after it had been subjected to strain by a 5% elongation of the strip and a subsequent annealing operation. It will be seen that the critical area has been materially improved and the circumferential line of division is much less noticeable.

FIGURE 3 discloses the same piece of material after having beensubjectedto an elongation of and a subsequent annealing step. The structure throughout the metal across the press stop is now of substantially homogeneous crystalline structure and is not subjected to the limitations in bending which were imposed on the material prior to treatment.

In the following examples which describe actual experiments performed, the essence of the invention is believed to be clearly set forth.

Example 1 A piece or extruded aluminum tubing containing a press stop and being of two inches outside diameter and wall thickness of 0.080 inch was strained by a 10% reduction of diameter by swaging. Subsequently, the swaged tube was annealed in an oven three feet long made from a split steel pipe and partially'insulated by a half section of rigid asbestos. The open ends of the oven were stuffed with asbestos'which provided an effective seal and also a support to keep the tube from contact with the oven.

A thermocouple attached to that part of the tube inside.

cycles. The test was stopped when a crack developed 18 inches away from the press stop area.

Example 2 An aluminum tube of an outside diameter of 3.0 inches and having a wall thickness of .100 inch was drawn down to a diameter of 2.76 inches, a reduction of 8%.

The press stops were annealed and the tube was subsequently swaged down over a core to a finished diameter of 2.665 inches, a total reduction of 11% of the starting diameter. The finished cable was then wound around a capstan and taken up on an inch drum reel (thirty times cable diameter); the cable was then rereeled onto another reel with an 80 inch drum and subsequently rereeled back to the original reel. Finally, the cable Was unreeled flat on the floor, but and subjected to a bend test and having four reverse bends over a form having a diameter twenty-six times cable diameter. The press stop was located at the critical point of bending.

No failure of the cable occurred.

The annealing step may also be carried out by electrical resistance heating. In this event electrical contact is made With the sheath on each side of the press stop region and. an electric current passed through the sheath. The resistance of the sheath causes its temperature to rise to eifect the annealing step. In one experiment conducted, a .415 inch OuD. sheath having a Wall thickness .025 inch was annealed by passing a current of 950 amperes through a 15 inch length of sheath containing a press stop region for a period of 10 seconds duration.

Other heating methods may be applicable to the present method.

It is believed that the present invention provides a simple, economical and practical method of overcoming the disadvantages which attended the use of aluminum tubing as a cable sheath when such tubing included press stops. Extra long lengths of aluminum sheathed cable may now be manufactured, in accordance with the present invention without splicing and without the disadvantages heretofore encountered.

The present specification has been drafted for the purpose of illustrating the present invention and the examples given are to be construed in an illustrative rather than limiting sense. Minor modifications in the method are contemplated within the scope of the subjoined claims.

What I claim as my invention is:

1. In a method of making an aluminum sheath for an electric cable comprising the steps of extruding a hollow sheath of the required length from a plurality of billets of aluminum, each billet forming one of a plurality of successive portions of the sheath, each portion being integrally joined to succeeding and preceding portions and having, at the .area Where one portion merges with an adjacent portion, a metallurgical circumferential line of division within the metal between a section of minute, granular, crystalline structure and a section having large, separate crystals, the further steps of first cold-working the sheath at and on both sides of the line of division, the cold-working being suflicient to give a plastic deformation of at least 4 /2% to at least one dimension of the sheath, and then annealing the strained sheath at a temperature above the recrystallization temperature but below the liquidus temperature of the aluminum to provide a substantially homogeneous crystalline structure in the aluminum sheath across the line of division.

2. The method of claim 1 in which the aluminum is cold-worked by drawing the sheath down to a smaller diameter along a length which includes the circumferential line of division and a portion of the sheath on both sides thereof.

3. The method of claim 2, in which the tube is swaged to reduce its diameter by an amount varying from 4% to 20%.

4. The method of claim 1, in which the annealing step is carried out at a temperature within a range of 600 F. to 1,00'0 F. fora period of from 30 minutes to five hours.

5. In a method of making an aluminum sheath for an electric cable comprising the steps of extruding a hollow sheath of the required length from a plurality of billets of aluminum, each billet forming one of the plurality of successive portions of the sheath, each portion being integrally joined to succeeding and preceding portions and having, at the portion of the tube that rested adjacent to an extrusion die during a dwell period when the press was being charged with a new billet, a press-stop characterized by a metallurgical line of division within the metal between a section of minute, granular, crystalline structure and a section having large, separate crystals, the further steps of cold-working the press-stop region by reducing the diameter of the tubing by an amount varying from 4 /2% to 20% through a drawing die, then annealing the said press-stop region at a temperature within the range of 600 F. to 1,000 F., for a period of from 30 minutes to five hours to provide a substantially homogeneous crystalline structure in the aluminum sheath across the region that previously had an undesirable circumferential line of division in the crystal structure of the metallic sheath.

References Cited in the file of this patent UNITED STATES PATENTS 2,067,076 Craighead Jan. 5, 1937 2,738,873 Emmerich Mar. 2 0, 1956 2,827,404 Klein Mar. 18, 1958 2,828,859 Emmerich Apr. 1, 195 8 FOREIGN PATENTS 711,859 Great Britain July 14, 1954 OTHER REFERENCES Physical Metallurgy of Aluminum Alloys by ASM, copyright 1949 (page 208 relied upon). Metal Industry, June 11, 1954, vol. 84, Aprillune 1954 (pp. 507-508 relied upon). 

1. IN A METHOD OF MAKING AN ALUMINUM SHEATH FOR AN ELECTRIC CABLE COMPRISING THE STEPS OF EXTRUDING A HOLLOW SHEATH OF THE REQUIRED LENGTH FROM A PLURALITY OF BILLETS OF ALUMINUM, EACH BILLET FORMING ONE OF A PLURALITY OF SUCCESSIVE PORTIONS OF THE SHEATH, EACH PORTION BEING INTEGRALLY JOINED TO SUCCEEDING AND PRECEEDING PORTIONS AND HAVING, AT THE AREA WHERE ONE PORTION MERGES WITH AN ADJACENT PORTION, A METALLURGICAL CIRCUMFERENTIAL LINE OF DIVISION WITHIN THE METAL BETWEEN A SECTION OF MINUTE, GRANULAR, CRYSTALLINE STRUCTURE AND A SECTION HAVING LARGE, SEPARATE CRYSTALS, THE FURTHER STEPS OF FIRST COLD-WORKING THE SHEATH AT AND ON BOTH SIDES OF THE LINE OF DIVISION, THE COLD-WORKING BEING SUFFICIENT TO GIVE A PLASTIC DEFORMATION OF AT LEAST 41/2 TO AT LEAST ONE DIMENSION OF THE SHEATH, AND THEN ANNEALING THE STRAINED SHEATH AT A TEMPERATURE ABOVE THE RECRYSTALLIZATION TEMPERATURE BUT BELOW THE LIQUIDS TEMPERATURE OF THE ALUMINUM TO PROVIDE A SUBSTANTIALLY HOMOGENEOUS CRYSTALLINE STRUCTURE IN THE ALUMINUM SHEATH ACROSS THE LINE OF DIVISION. 